Froth flotation for the separation of a mineral from its ore may be carried out by hydraulic-pneumatic means using vertically elongated cells or columns. A feed of an aqueous pulp of suitably ground ore and conditioned with reagents as required is supplied at a feed point situated some distance below the top of the column. A gas, usually air, is sparged into the bottom portion of the column. Reagents such as collectors, frothers, activators, depressants and the like may be used. The sparged air bubbles rise in the column, adhere to conditioned mineral particles and the mineral collects in a froth layer which forms in the upper portion of the column, while the gangue materials or tailings displace downwardly in the column into its lower portion. The gangue materials are removed as tailings from the bottom of the column. The froth layer wherein the mineral becomes concentrated also contains an amount of entrained gangue materials. To improve the separation of gangue from mineral the froth layer is washed with an amount of washwater distributed onto or in the froth layer. The washed froth overflows from the top of the column and the mineral is recovered as a concentrate.
In order for the separation to be carried out in an efficient manner it is necessary to control one or more parameters which may include both physical and chemical variables such as the flows of feed, concentrate and tailings, reagents additions, pulp level and density, air and wash water additions, pH, and the pulp/froth interface level.
One of the important parameters is the pulp/froth interface level. Two separate zones exist in a flotation column, namely, an upper or cleaning zone essentially containing the froth layer and a lower or collecting zone wherein gangue materials sink to the bottom and the rising air bubbles attach to the conditioned mineral particles taking them upward into the froth. The two zones are separated by a rather distinct interface. The feed slurry is added to the column at a position below the interface that exists between the cleaning and collection zones. If the level of the interface is too high the volume allowed for the froth layer may be too small and washing may not be carried out effectively resulting in a reduction of the concentrate grade. If the level is too low the volume of the collection zone is reduced, froth stability becomes limiting and recovery of mineral will be reduced. Another important variable is the amount of washwater added that is necessary to enhance the removal of gangue entrained with the mineral in the froth layer. Too little washwater gives insufficient washing tending to lower the grade, while too much washwater tends to reduce the recovery.